Convenient and inexpensive methods for determining pressure maps of surfaces, particularly aerodynamic objects, have long been sought. Static pressures over the surface of an aerodynamic body are presently determined using pressure orifices or pressure taps, which are drilled into the surface of the body and connected via tubing to multiplexed electronic pressure scanners external to the surface of interest. Hundreds or even thousands of pressure orifices may be required to map an entire surface. Since the pressure orifices are usually separated by a significant distance, a continuous pressure map is not achieved. The current method for determining pressures between the orifices is to interpolate them by computational fluid dynamics (CFD). See Erisman, A. M. and Neves, K. W., Sci. Am. 257, 163 (Oct. 1987) and Jameson, A., Science, 245, 361 (1989).
Wind tunnel experiments provide the data to validate CFD models. Data is also used to gain an understanding of flow so as to guide development of new CFD models. A method for collecting continuous pressure data in wind tunnels would provide the information needed to eliminate some of the uncertainty in CFD models.
Several methods have been used in an attempt to provide a convenient method for determining static pressure on aerodynamic surfaces. Digital holographic interferometry is one process whereby interference fringes are counted, and the distances between fringes are processed into pressure distribution information of the surface of an airfoil. See Merzkirch, W., Flow Visualization (Academic Press, New York, 1974). This technique, however, is valid only for symmetrical airfoils.
In 1980, Peterson and Fitzgerald (Peterson, J. I. and Fitzgerald, R. V., Rev. Sci. Instrum., 51, 670 (1980)) proposed oxygen quenching of fluorescent dyes for flow visualization in a wind tunnel. In their experiment, the luminescent dye was adsorbed onto silica particles. The coating was rough and adherence was a problem. No attempt at quantitation was made.
The methods and compositions described herein are based on molecular photoluminescence. Luminescence is a broad term which encompasses both fluorescence and phosphorescence. Electromagnetic radiation in the ultraviolet or visible region is used to excite molecules to higher electronic energy levels. The excited molecules lose their excess energy by one of several methods. Fluorescence refers to the radiative transition of electrons from the first excited singlet state to the singlet ground state (S.sub.1 .fwdarw.S.sub.o). The lifetime of fluorescence is relatively short, approximately 10.sup.-9 to 10.sup.-7 seconds. However, intersystem crossing from the lowest excited singlet state to the triplet state often occurs and is attributed to the crossing of the potential energy curves of the two states. The triplet state so produced may return to the ground state by a radiative process known as phosphorescence. Phosphorescence is the radiative relaxation of an electron from the lowest excited triplet state to the singlet ground state (T.sub.1 .fwdarw.S.sub.o). Because the transition that leads to phosphorescence involves a change in spin multiplicity, it has a low probability and hence a relatively long lifetime (10.sup.-4 to 10 seconds). Also, due to the lower energy of the triplet state, the wavelength of phosphorescence is longer than the wavelength of fluorescence. Herein, the term "sensor" is used to refer to luminescent molecules.
In one embodiment of the present invention, a phosphorescent porphyrin is coated on the surface of an object. The quenching of phosphorescence emitted by the porphyrin upon excitation is used to quantitatively measure the static pressure on the surface of the object.
Porphyrins are macrocyclic tetrapyrrole structures, some of which are known to phosphoresce when exposed to specific frequencies of light. See Falk, J. E., Porphyrins and Metalloporphyrins, Vol II (Elsevier, Amsterdam) chap. 1, 1964; and Gouterman, M. in The Porphyrins, Vol III, Physical Chemistry, Part A, Ed. D. Dolphin (Academic Press, New York), chap. 1, 1978). This phosphorescence is also known to be quenched by oxygen. See Cox, G. S. and Whitten, D. G., Chem. Phys. Let. 67, 511 (1979) and Rossi, E. et al., Photochem. Photobiol. 42, 447 (1981).
The oxygen quenching properties of platinum porphyrins have been used for the determination of oxygen in vivo. For example, U.S. Pat. No. 4,810,655 is directed to methods and compositions for measuring oxygen concentration, particularly for monitoring oxygen in the blood with a fiber-optic catheter.
In one important aspect of the present invention, the inventors have recognized that there is a problem with prior techniques of pressure measurement based on oxygen quenching of luminescence. In particular, the phosphorescence of certain porphyrin derivatives has a temperature dependency in addition to a pressure dependency. Accordingly, pressure measurements based on a coating containing a single sensor of this type can be in error if there is a fluctuation in the temperature. Previously, it has been reported that there was no temperature dependence of the brightness of a particular luminescent indicator coating. See Ardasheva, M. M., et al., Zhur. Prik. Mek. 4, 24-30 (1985), English translation. To the contrary, the present inventors have found that for the systems reported herein, the temperature dependency of luminescence is an effect which must be corrected for when temperatures on the surface of an object vary more than approximately 0.5.degree. C.
There are also previous reports to the effect that, independent of pressure measurements, temperature sensitive phosphors have been used to measure surface temperature of aerodynamic surfaces. See, for example, Bradley, L. C., Rev. Sci. Instr. 24(3), 1953; and Baker, H. D. et al., Temperature Measurement in Engineering, Vol II (Omega Press, Stamford, Conn.) 190-191 (1960). None of these prior reports have incorporated two different types of sensors as disclosed herein into a single film for improved pressure measurements of surfaces.